1. Field of the Invention
The present invention relates generally to a method of recharging secondary lithium based batteries and more particularly to a method and apparatus for enhanced cycle life for secondary lithium batteries resulting from recognition of potential shunting, electrolyte decomposition, and unequal states of charge between individual cells of the battery, all of which are conditions which may occur during recharging of lithium batteries.
2. Background and Description of the Related Art
As miniaturization and power saving have proceeded in the field of electronics, secondary batteries using alkaline metals such as lithium have attracted attention. Rechargeable lithium batteries operating at room temperature offer several advantages compared to conventional aqueous technologies, including: higher energy density (up to 150 Wh/kg, 300 Wh/L); higher cell voltage (up to about 4 V per cell); and longer charge retention or shelf life (up to 5 to 10 years). These advantages result in part from the high standard potential and low electrochemical equivalent weight of lithium. Rechargeable lithium batteries can be classified into essentially five different classifications. The first type is a solid-cathode cell which uses intercalation compounds for the positive electrode, a liquid organic electrolyte, and metallic lithium as the negative electrode. The second type is a solid-cathode cell which uses intercalation compounds for the positive electrode, a polymer electrolyte, and a metallic lithium negative electrode. The third type uses intercalation compounds for both the positive and the negative electrode and a liquid or polymer electrolyte and are commonly referred to as lithium ion cells. The fourth type are inorganic electrolyte cells which use the electrolyte solvent or solid redox couple for the positive electrode and lithium metal for the negative electrode. The fifth general type are cells include lithium alloy anodes, liquid organic or polymer electrolytes, and a variety of cathode materials, including polymers.
One of the problems with using rechargeable lithium cells is the reactivity of metallic lithium with the electrolyte. This reaction is not thermodynamically stable in most organic electrolytes. Accordingly the surface of the lithium is normally covered by a film of the resulting reaction products. Each time lithium is stripped and replated during discharge and charge, a new lithium surface is exposed and passivated with a new film of reaction products, consuming both lithium and electrolyte.
Another problem is electrolyte decomposition. Electrolyte decomposition often occurs if the cell's voltage during charging exceeds the maximum voltage of the cell. In this situation, the rate of electrolyte decomposition is directly proportional to the magnitude by which the voltage of the battery exceeds the maximum voltage of the battery or overvoltage of the battery. The overvoltage is the difference between the voltage across the battery and the ultimate electrolyte decomposition voltage or maximum voltage of the battery (V-V.sub.MAX). Decomposition of the electrolyte results in increased internal resistance in the battery and ultimately battery failure. A principle cause of electrolyte decomposition is the charging of the battery under constant voltage after achieving of V.sub.MAX. Any time the pre-determined maximum voltage is achieved or exceeded, electrolyte decomposition occurs. Under constant current charging, this condition of exceeding the maximum voltage of the battery can occur for a relatively long period of time with resulting severe electrolyte decomposition. Conversely, if the battery is charged at a rate which is lower than the maximum voltage to prevent electrolyte decomposition, the battery will be undercharged and cell cycle life will diminish.
Traditionally, in order to achieve reasonable cycle life, a three to five fold excess of lithium is required in rechargeable lithium cells. Beyond the increased reactivity caused by a three to five fold increase in reactive lithium, the lithium which is electroplated onto the metallic surface during recharging is much more porous than the original metal. The porous lithium results in a larger surface area of lithium being deposited and exposed to the electrolyte. In addition, the freshly formed lithium that is plated during recharging is highly reactive, as well as more susceptible to forming dendrites which can short circuit or electrically shunt the battery. Since the reaction of metallic lithium with the electrolyte is exothermic, even minimally exothermic lithium-electrolyte reactions can rapidly increase the internal temperature of the battery after a plurality of charge-discharge cycles.
Another problem with charging rechargeable lithium based batteries is unequal or different states of charge between individual cells within the battery. The difference in cells capacity of the battery will often result in overcharge or overdischarge of the individual cells. This is a particular adverse consequence when using lithium based batteries since they are very sensitive to overcharge and overdischarge. An example of a particularly adverse consequence of cell overdischarge is the corrosion of the copper current collector on the negative electrode. This results in an increase in the internal resistance of the battery.
Overdischarge also may cause an irreversible change in the crystal structure of the positive electrode (MnO.sub.2, CoO.sub.2, or NiO.sub.2) due to deep cathodic polarization of the positive electrode. Other battery chemistries (i.e. nickel based batteries utilizing water electrolyte) can be overcharged to avoid unequal states of charge in individual cells because they employ a "chemical pathway" to equalize the individual cells. For example, nickel based batteries can rely on an oxygen cycle to equalize individual cells and convert excess electrical energy into heat. However, lithium based batteries do not have an oxygen cycle, and typically are equalized by monitoring each cell's voltage during charging, and charging or discharging a particular cell to equalize the individual cells. Simply monitoring the voltage of the individual cell, however, does not take into consideration the internal resistance of the cell, which may cause a cell or group of cells to appear to need adjusted when in reality there are no differences in the state of charge of the individual cells.
In recent years a new generation of rechargeable batteries have been introduced employing an intercalated carbon material as the negative electrode instead of metallic lithium. Intercalation of lithium ion occurs during the discharging process and de-intercalation of lithium ion occurs during the charging process. The use of a carbon matrix is designed to avoid the problems associated with the earlier metallic lithium batteries by eliminating electroplating of metallic lithium and thus prevent dendrite formation and minimize the chemical reaction of lithium metal and the electrolyte by eliminating the availability of lithium metal. However, in many instances the use of the carbon matrix only initially avoids the problem of reactive surface area due to electroplating or shunting. A cause of this may be electroplating of lithium metal on the external surface of the carbon matrix. As a result, these newer generation batteries may face many of the same cycle file and safety concerns encountered by the earlier generation lithium batteries.
U.S. Pat. No. 5,481,174 assigned to Motorola attempts to address the formation or appearance of shunts by regulating the average current, the current being dependent on a control time of reaching a pre-selected maximum voltage. Although this approach does help alleviate the decomposition of the electrolyte, it requires discharging of the cell and does not address the role of a time factor for current adjustment.
U.S. Pat. No. 5,442,274 teaches the use of hysteresis or repetitive peak and through charging prior to constant voltage charging. During peak charging, the battery is charged by a constant current or quasi-constant current, and during the trough of the charging wave, the battery charging process is either suspended or the charging current is reduced from the level at peak charging. This is a simplified charging process which does not adequately address the electrolyte decomposition concerns associated with lithium batteries.
U.S. Pat. No. 4,736,150 discusses the benefits of pulse charging to prolong the cycle life for a lithium molybdenum disulfide (Li/MoS.sub.2) batteries. Again this method does not address the cycle life problems or issues confronted when using lithium based rechargeable batteries.
The Benchmark Microelectronics reference discloses a hysteresis charging profile which uses a pulse current between maximum and predetermined minimum voltages. This technology which regulates current through frequency modulation avoids current tipper and can not be acceptable for lithium based batteries. The constant current amplitude creates problems associated with diffusion resistance as a result of ion transport problems inside the solid positive electrode. This resistance leads to an increase in the battery voltage and therefore to battery undercharge.
A need exists in the art for a method of charging lithium based batteries which improves cycle life, efficiency, and safety of the batteries.